Acetylene is a commercially important gas which is widely used and extensively distributed. Because acetylene is thermodynamically unstable with respect to its decomposition into carbon and hydrogen, it will decompose explosively when subject to a suitable ignition source. An explosive decomposition can occur even in the absence of an oxidizing agent such as air or oxygen. For this reason, elaborate precautions must be taken to insure that acetylene is not exposed to ignition sources during shipment or storage.
Acetylene is commonly shipped in special containers in which the acetylene is dissolved in a solvent, such as acetone or dimethylformamide, and this solution is dispersed in a porous, inert filler heat sink material, such as calcium silicate. These containers are commonly fitted with flash arrestors and fusible plugs to limit the danger of explosion. Despite these precautions, the pressure of acetylene gas which can be tolerated over the acetylene solutions in these containers is limited to about 15 atm at 23.degree. C. At pressures higher than about 15 atm, the combined thermal mass of the solution, the solvent and the inert filler heat sink material is not great enough to prevent an explosive decomposition from propagating through the container once it has been initiated.
A similar situation exists in pipelines which are used to transport acetylene gas. Acetylene is transported as a gas in such pipelines at pressures which must be limited to less than about 1.5 atm. To minimize the danger of explosion or to limit the extent of an explosion, these pipelines are often filled with inert material, such as raschig rings, and they are fitted with flash arresting devices to limit the propagation of explosions.
Common sources for the ignition of acetylene are: sparks, flashbacks from torches or flames, and hot walls of containers or pipes produced by the inadvertent application of external heat sources. In general, explosive decompositions are initiated in the gas phase. In acetylene containers, this decomposition can then propagate through the solution and filler resulting in rupture of the container. It is desirable, therefore, to insure that the gas phase acetylene be rendered as stable as possible. This can be accomplished, for example, by reducing the pressure of the acetylene gas. p It is known that the partial pressure of acetylene gas at a given total pressure can be effectively reduced while at the same time providing an additional heat sink if an inert gas, such as nitrogen or carbon dioxide, is mixed with the acetylene. Thus if a mixture at a total pressure of 10 atm consists of a partial pressure of acetylene of 9 atm and a partial pressure of N.sub.2 of 1 atm, the ignition behavior of such a mixture is nearly identical to that of pure acetylene at a pressure of 9 atm.